FindOpenGL: unset OPENGL_USE_<C> variables on exit

If the OpenGL find module is called twice in a project, first with a
component <C> then the second time without, the second call will use the
value of the OPENGL_USE_<C> variable from the first call.

This has been seen to cause incorrect results in a project using Qt where the
first call is part of the project's own logic and the second call happens within
a call to Qt 6.12.0's Qt6 find module.

Fixes: #28246

Modules/FindOpenGL.cmake

@@ -901,3 +901,8 @@ set(OPENGL_INCLUDE_PATH ${OPENGL_INCLUDE_DIR})
 
 mark_as_advanced(${_OpenGL_CACHE_VARS})
 unset(_OpenGL_CACHE_VARS)
+
+foreach(component ${OpenGL_FIND_COMPONENTS})
+  string(TOUPPER ${component} _COMPONENT)
+  unset(OPENGL_USE_${_COMPONENT})
+endforeach()